The involvement of human prolactin (hPRL) in breast cancer has been recently established based upon its autocrine/paracrine proliferative effects in mammary tumor epithelial cells. More importantly, recent evidence suggests that hPRL may constitutively activate oncogene HER2/neu through activation of JAK2 kinase. Although the molecular events involved in this cross activation is not well understood, it is believed that constitutive phosphorylation of HER2/neu contributes to poor prognosis and unsatisfactory clinical response of HER2/neu positive breast cancer to anti-HER2/neu monoclonal antibody therapy. The purpose of this two-year pilot study is to identify the alteration of gene expression in the mammary gland resulted from the interaction between hPRL and HER2/neu, more importantly between a hPRL antagonist (G129R) and HER2/neu, using bi-transgenic mouse models. We are especially interested to test the hypothesis that G129R can be used as a chemopreventive agent to prevent or delay the onset of HER2/neu initiated breast cancer and identify those genes involved in this process for future studies. Our specific aims are (1) to establish bi-transgenic mice by cross-breeding male mouse metallothionein I promoter (MT) hPRL transgenic mice (MT/hPRL) and MT/G129R transgenic mice with female MMTV/neu transgenic mouse to generate bi-transgenic lines (MT/PRL+MMTV/neu and MT/G129R+MMTV/neu), (2) to monitor the rate of breast tumor formation in bi-transgenic mice to determine the role of hPRL in HER2/neu induced breast tumor and the efficacy of potential chemopreventive effects of G129R, (3) to use commercial cDNA microaray services to identify alterations in gene expression in the mammary tissue of the bi-transgenic mice to identify genomic signatures of hPRL, G129R and HER/neu interaction. We hope that the results from this pilot study will not only contribute to a better understanding of the HER2/neu positive breast cancer but also lay a solid foundation for further studies to define therapeutic targets for HER2/neu positive breast cancer as well as to develop HER2/neu specific tumor inhibitors.